A rotary kiln is an apparatus primarily used for drying which consists of a slowly rotating cylinder which is slightly inclined relative to a horizontal plane. The length of the cylinder may range from 4 to more than 10 times its diameter. A wet solid product is fed into the higher end of the cylinder. The solids progress through the cylinder by virtute of the rotation and slope of the cylinder for discharge as a dry or semi dry product at the cylinder lower end. Hot gas flows through the cylinder and interacts with the solids to cause drying. The rate of solids flow may increase or decrease depending on whether the gas flow is co-current or counter-current to the solids flow. Rotary kilns operate at fairly high temperatures and are typically lined with insulating block and/or refractory brick. The means for supplying the hot gas may vary. Some kilns utilize direct firing of a combustible such as natural gas with the combustion products in direct contact with the solid product. If contamination of the product with combustion gases is undesirable, indirect gas or oil fired air heaters may be employed to achieve temperatures in excess of steam temperatures. Of course, gasses other than air may also be used.
The method of feeding the kiln depends upon the solid product characteristics and on the location and type of upstream processing equipment. When the feed comes from above, a chute extending into the cylinder may be employed. For sealing purposes, or if gravity feed is not convenient, a screw feeder is normally used. Typically, auxiliary equipment may be associated with the kiln. For example, the exiting gas may pass through an apparatus for removing entrained dust prior to discharge and/or through apparatus for recovering heat.
When utilizing a counter-current rotary kiln, where gases flow in a direction opposite to the direction in which the product is preceding, it is common for dust or solid particles to become entrained with the gas flow. With such rotary kilns, as the gas is redirected at the kiln feed end, commonly known as the "feed breech", to exit the kiln, the entrained particles strike the kiln wall, become disentrained and accumulate. Such accumulations in the feed breech require periodic kiln shut-downs for clean-out through access holes. Such shut-downs require time for the kiln to cool down, removal of insulation, removal of the accumulated material, etc., which is quite disruptive of continuous operations.